According to a purported leaked T-Mobile document obtained by TMoNews, the carrier will offer an unlimited contract free mobile calling add-on for $10 a month starting April 4th. The new feature does not require a contract, and can be added or removed from a particular plan at will. Once added, customers will be free to call any US mobile without having to think about minutes.

The move comes as T-Mobile pushes to reposition itself as a cheaper alternative to larger rivals, as Sprint and AT&T customers have actually enjoyed unlimited mobile-to-mobile calling for some time now. We just reported that T-Mobile is expanding its coverage, developing a 4G LTE network compatible with the upcoming iPhone 5, so the carrier cannot afford to slash data prices to attract customers.

The new feature is set to be introduced in the United States and Puerto Rico on April 4th. More details will probably be announced soon. Stay tuned.